OZONATION AND GRANULAR ACTIVATED CARBON FILTRATION
THE SOLUTION TO MANY PROBLEMS
Extract from the
PROCEEDINGS
of the
FIRST AUSTRALASIAN CONFERENCE
of the
INTERNATIONAL OZONE ASSOCIATION
DOWN UNDER ‘96
Sydney, February 1996
OZONATION AND GRANULAR ACTIVATED CARBON FILTRATION
THE SOLUTION TO MANY PROBLEMS
J. Aeppli and P. Dyer-Smith
Ozonia Ltd
Duebendorf, Switzerland
Abstract
This paper explains how ozone is used in the purification of surface water to potable
water both as the first treatment stage and as an intermediary stage for the reduction of organics and as a disinfectant. Taste and odour of the water are improved by
the oxidation of both organic and inorganic impurities by ozone. Products resulting
from algae metabolisms are shown to be easily oxidised by ozone. The oxidation
mechanisms of these oxidation reactions are explained. The importance of the GAC
(Granular Activated Carbon) filtration stage after ozonation for the removal of the
by-products is highlighted and the complex way in which adsorption takes place is
fully described.
Introduction
Ozone is the strongest oxidising agent, which is actually applied in many water treatment plants, without the disadvantages of chlorine. Mainly in Europe ozone has already displaced chlorine in almost
all water treatment plants for preoxidation, main disinfection and degradation of organics. The same
is happening in the paper pulp bleaching field. This tendency is on going, because ozone has many
advantages in comparison with chlorine, without having the disadvantages of chlorine. These are the
main reasons why ozone is so successful today.
Many different effects have to be mentioned and most of them are dependent on where the ozone is
introduced and how much is applied. Two major places of introduction with different aims are realised
today in water treatment processes as follows:
•
•
Ozone as preoxidation
Ozone as mainoxidation
For preozonation the main goals can be summarised as follows:
•
•
•
•
•
•
•
•
Removal of soluble iron and/or manganese by oxidation
Decoloration
Removal of taste and odour
Improved microflocculation
Reduced trihalomethanes
Decrease the trihalomethane-precursors
Satisfy the sponteanous ozone demand
Oxidation of inorganic substances such as cyanides, sulphides and nitrites.
1
For preozonation normally low doses are applied and a short reaction time in the order of 1 to 2 minutes.
This is in accordance with the fast reaction taking place at this stage. Usually no or only small ozone
residuals are determined in these applications.
•
•
•
•
•
•
•
•
The main goals for a mainozonation can be summarised as follows:
Bacterial disinfection
Inactivation of viruses
Oxidation of organic compounds, e.g. phenols, detergents, pesticides or other hardly biodegraded
substances
Conversion of COD to BOD
Reduction of DOC, if a biological filtration step is installed downstream, e.g. Granular Activated
Carbon (GAC), Anthracite or Slow Sand Filter
Reducing the necessary dose for the network protection (chlorine, chlorine dioxide)
Oxidation of complexes, such as EDTA and NTA.
A typical reaction time is in the order of 4 minutes or higher and the reaction rates are slow. To have a
proper disinfection the so called c.t-value is applied.
This is achieved by maintaining an ozone residual of 0.4 mg/l during 4 minutes resulting in a c.t-value
of 1.6 mg.min./l. It is known from practical experience that this would reduce the bacterial counts as
follows:
3 decade reduction for Giardia cysts
and a
4 decade reduction for Enteroviruses.
Additionally, hydrogen peroxide can be used to accelerate the removal of pesticides or other persistent
organic compounds. In some cases the residual ozone is removed by the addition of hydrogen peroxide
or simply by stripping with air.
Occurrence of taste and odour
The sensation of taste and odour are complementary. With the 3000 to 10 000 taste buds in the mouth,
we are able to detect more readily the inorganic constituents in the water, while the sense of smell is
more useful to detect organic constituents.
Taste and odour problems may occur in any potable water and are generally independent of season.
Primarily these problems are associated with:
•
•
•
•
The raw water
The treatment method
The distribution system
Or any combination of these three.
The nature of the problem can be either:
•
•
•
Inorganic compounds (e.g. Al, Fe, Mn, Zn, H2S, CrO)
Organic compounds from natural sources (e.g. algae, humic acids)
Contamination of raw water by discharges from industry or agriculture.
Most inorganic substances exert an unpleasant taste at concentrations much lower than those required
for acute toxic effects. For pesticides however the threshold taste value for detection is higher than the
recommended guideline values.
2
Water treatment plants can convert weak odour substances into substances with a very intense odour.
Example of this are amines and phenols, which will be converted to chloramines and chlorophenols by
any chlorine treatment (pre- and/or post-chlorination).
Aquatic organisms have metabolic activity and can release some unpleasant substances. Two main algae
species that are the origin of such substances are:
•
•
Blue green algae (Cyanophycea)
Actinomycetes
The most dominant metabolites are Geosmine and Methylisoborneol (MIB), for which the structural
formulas are given in Figure 1. Threshold levels for Geosmine are in the order of 10–20 ng/l giving an
earthy odour.
CH3
CH3
CH3
7
OH
1
CH3
2
6
4
5
OH
CH3
3
CH3
(2 - exo -hydroxy - 2 - m thylbornane
(Trans - 1, 10 - dim thyl
ou 1, 2, 7, 7 - t tram thyl - 2 - norbornanol)trans - 9 d calol)
C11 H20 O
C12 H22 O
Figure 1: Molecular structures of 2-Methylisoborneol and geosmine
Further examples are the THM precursors which will be converted to THMs by the use of chlorine
(prechlorination, mainchlorination or addition of chlorine at the end of the treatment process as network
protection). Besides their unpleasant odour these substances are known to be carcinogenic.
Origin of odour
Algae not only synthesise their own matter but also release chemical mediators (metabolites) into the
aquatic environment. High release rates of such chemicals appear especially during algae blooms or
when the conditions of the nutrients are not in harmonic equilibrium.
According to the algae type the description of the odours are manifold such as:
– grassy, musty, garlic, fishy, oily, septic, bitter, spicy, cucumber, etc.
Furthermore, some of these chemicals are known to form trihalomethane precursors, which will cause
taste and odour problems in drinking water treatment plants either directly or enhanced when chlorine
is used for treatment purposes.
As far as taste and odour compounds are concerned they belong to a group of volatile organic compounds, with a molecular mass of up to 200 g/mole. Most of them are unknown but recently it has been
3
possible to identify some of the organic compounds released by the blue algae Anabaena.
As an example of this the plot of a gas chromatographic separation for the blue algae Anabaena is given
in Figure 2 together with a list of 40 identified substances (see Table 1).
2
mV
5
4
3
6
9
11 12
14
10
15
16
13
25
20
30
17
29
18
27
33 35
36
28
26
21
40
19 23
22
24
20
34
39
1
0
31
40
60
80
32
37 38
41
100
120
¡C
Figure 2: Seperation of volatile compounds by GC from an Algae Bloom of Anabaena in LAke Constance (24m glass capillary,
UCON 50 HB 1500, Carrier Gas H2)
For the scope of this paper it is important to recognise that most of these compounds are either unsaturated olefins or aromatic compounds and therefore easily oxidised by ozone.
2
3
4
5
6
9
10
11,12,17,18
14
15
16
20
22
23
benzene
cis-1,2 dichloroethylene
pentanone – 3
trichloroethylene
chloroform
tetrachlorethylene + toluene
dimethyldisulfide
monoterpene (isomeric)
ethylbenzene
p-xylene
m-xylene
o-xylene
n-undecane
n-propylbenzene
Table 1: Identified compounds from Anabaena [3]
4
24
25
26
27
28
30
31,32,33&37–39
35
1,4 – ethylmethylbenzene
1,3 – ethylmethylbenzene
limone
mesitylene
1,2 – ethylmethylbenzene
pseudocumene
C4 – benzene
39
34
36
hemelitene
indene
40
p-dichlorobenzene
The applied ozone dose for the elimination of such odours is generally low, e.g. around 2 mg/l (Figure
3, curve A) as shown by the example of the River Seine water. During certain periods other organic
matter appear in the raw water and the dose-effect curve is different (Figure 3, curve B). This curve
corresponds to the evidence of odour compounds such as aldehydes being present.
odour index
6
5
B
4
additional organic
compounds (aldehydes)
3
2
A
1
normal case
Ozone mg. 1-1
0
Figure 3: Influence of the ozone dose to the odour index
Oxidation of organic compounds
To understand the importance of the combination of ozone and GAC filtration it is necessary to explain
in some words the reaction mechanism and their by-products.
The most significant reactions of ozone with organic matter are based on the cleavage of the carbon
double bond, which acts as a nucleophile or a specie having excess electrons.
Ozone will always react in two different mechanisms:
•
•
Direct reaction of ozone with organic compounds
Reaction by free hydroxyl radicals.
This reaction model has been described by Hoigné in 1975 with the following sequences:
Ozone reaction mechanism
(Hoign
1975)
O3 stripped
O3 addition
M
O3 dissolved
+OH
OH¡
termination reaction
(scavenger)
+Si
O
oxid
direct reaction
(M = Solute)
or R¡
+M
R¡
radical type reaction
5
The consumption of ozone by direct reaction can be written as a pseudo first order reaction if the solute
concentration is higher than the ozone concentration:
d [Ο 3]
 = ko3 [M] • [O3] t
dt
ko3
:
reaction rate constant (l.mole -1.s -1)
M
:
solute concentration (mole.1-1)
O3
:
ozone concentration (mole.1-1)
Depending on the chemical group of compounds a great range of reaction rate constants have been
published.
Reaction rate constants of ozone and hydroxyl radicals are given in Table 2.
Compound
k, in L.mole -1.s -1
O3
°OH
Olefins
1000 to 4.5 x 105
10 9 to 1011
S-organics
10 to 1.6 x 103
10 9 to 1010
Phenols
103
10 9
N-organics
10 to 10 2
10 8 to 1010
Aromatics
1 to 10 2
10 8 to 1010
Acetylenes
50
10 8 to 10 9
Aldehydes
10
10 9
Ketones
1
10 8 to 10 9
Alcohols
10-2 to 1
10 9 to 1010
Alkanes
10-2
10 6 to 10 9
Table 2: Reaction rate constants k of ozone and hydroxyl radicals with organic compound
It is important to notice that the direct reaction depends strongly on the chemical nature of the organic
compound, whereas the radical type reaction is more or less independent. Furthermore, the reaction rate
constant for the radical type reactions are a few orders of magnitude higher when compared to those of
the direct reaction.
The mechanism of the cleavage of a double bond can be simplified in the following manner (Figure 4)
In this reaction chain the species of aldehydes or ketones appear twice as reaction products, the carboxylic acids only once. A further oxidation of these products will not take place easily because the reaction
rate constants are low (direct reactions).
The aldehydes as well as the ketones are known to be nutrients for bacteria. It is therefore important to
have a biological step after a mainozonation, as the formation of aldehydes and ketones are normally
more important than after a preozonation.
6
C = C
+ O3
+
C
+ H2O
+HOH
Hydroxyhydroperoxide
O
O
-
+
(unstable)
O-C
(ketone or
aldehyde)
O-OH
C
OH
- H2O2
-HOH
carbonic acid R
C
O
R
OH
H
C = O
ketone or
aldehyde
Figure 4: Direct oxidation reaction
The role of a GAC filter after ozonation in water treatment
A GAC filter after an intermediate ozonation has many purposes:
•
•
•
To destroy the residual ozone in the water fed to the GAC filter – this takes place in the top few
centimetres of the GAC filter
To remove chemical compounds or ozonation by-products by adsorption
To degrade such substances by biological activity on the surface of the GAC by bacteria.
In a GAC filter different competitive processes take place simultaneously:
•
•
•
•
Fast adsorption
Slow adsorption
Biological effects and
Biological effects enhanced by ozonation.
Between the fast and the slow adsorption there is competition. New GAC will first adsorb a lot of
weakly adsorbed compounds, e.g. alcohols, ketones, aldehydes, acids, aliphatics and colloids. These
compounds will then be displaced from the active carbon surface by more strongly adsorbed pollutants,
e.g. aromatics, chlorinated aromatics, pesticides, chlorinated nonaromatics, and high molecular weight
hydrocarbons. The displaced, weakly adsorbed material will be redsorbed further down in the filter.
This phenomenon is called the “chromatographic effect”.
Three types of adsorption can be distinguished for a GAC filter:
•
•
•
Exchange adsorption (electrical attraction of the solute by the adsorbent)
Physical or ideal adsorption by weak van der Waals forces
Chemisorption or chemical adsorption (chemical reaction of the adsorbate with the carbon).
In the water treatment application the primary mechanism is the physical adsorption, which is reversible
followed by the chemisorption, which is generally considered as irreversible.
A GAC filter offers an excellent surface for biological activity. The rough surface provides numerous
good places for attachment. Such biological activity has become evident in full-scale plants, showing
that the amount of organic carbon removed is far beyond that which can be removed by adsorption alone
(see Figure 5).
DOC
Raw water
Slow adsorption
+O3
Fast adsorption
Biological effect
Effect by ozone
Final water
non adsorbable matter
source Y. Richard (1)
Temps
Figure 5:Elimination of organic matter
It is well established that an ozone step will produce aldehydes and ketones by oxidation of the carbon
doublebonds. These products are nutrients for the bacteria which are always present in a distribution
system (tolerance value in Switzerland: up to 300 per ml). If these compounds are not removed during
the treatment process, they will enhance the regermination in the network system rapidly and dramatically. Complaints from the health authority will be the consequence. This situation has to be avoided
and is so by the introduction of an appropriate biological treatment step, e.g. GAC filter or slow sand
filter. If such biological filters are present after the main-ozonation the same organic compounds will be
biodegraded on the surface of the filtermedia (Figure 6).
An additional measure to avoid a bacterial regrowth problem in the distribution network is the dosing
of an oxidising agent at the end of the treatment plant, e.g. chlorine, chlorine dioxide or chloramine. If
such a biological filter is not installed, the dosing of the oxidising agent has to be increased to avoid any
bacterial problems in the distribution system.
With a new GAC filter rapid adsorption is dominant and nearly all organic material can be adsorbed.
According to the adsorption capacity of the filter and with increasing running time, the rapid adsorption
will decrease in favour of a slow adsorption. At the same time a biological activity is started and will
additionally reduce the organic matter (Figure 6).
If a pre and/or main ozonation is introduced befor GAC filtration a further enhancement of the biological activity takes place. A further reduction of the DOC will be achieved while a part of the by-products
will be adsorbed on the GAC and/or consumed by bacteria. Such a combination improves the final
bacterial growth, 1-16
macropore, 50-100
GAC particle
micropores, 0.05
average size
,
Figure 6: Schematic representation of a GAC particle showing pore size and bacterial growth
water quality [1].
Practical Experience in Water Treatment Plants
As a practical example the build-up of aldehyde has been measured at the Zurich’s water supply treatment plant Lengg after the introduction of the two-stage ozonation (2).
Figure 7 shows the cumulative graphs of aldehydes after preozonation, after rapid filtration, after main-
ng/l
1200
1000
C12 C8 = carbon chain length
C10
C8
C8
800
C7
C6
600
400
200
0
raw water
after preozonation
after
after
rapid
intermediate
filtration ozonation
Figure 6: Schematic representation of a GAC particle showing pore size and bacterial growth
after
GAC
ozonation and after GAC filtration. Both ozonation steps contribute to the build-up of the aldehydes
which show chain length between five and twelve C-Atoms. The formation of the aldehydes is dependent on the ozone dose. Higher doses will increase the formation more or less linearly. After the GAC
filter no aldehydes can be detected even by the very sensitive analytic method of gas chromatography.
Experiences in the lake water plant Lengg at Zurich have shown the time dependence of the different
effects. From these experiences it can be concluded that an operating equilibrium is reached after about
3–4 months (Figure 5). This result is valid for either new or reactivated GAC [2].
At this water treatment plant, the GAC has only been reactivated twice, the first time in 1980 and a
second time in 1986. Since then no further reactivation took place with the justification that the GAC
filtrate quality is stable due to biological activity.
Degrémont implemented the combination of Ozone-GAC technology in the seventies and this technology has since been introduced successfully in many water treatment plants. It is the most effective
technique to eliminate organic matter. If most of the assimable organic matter (AOC) is eliminated by
the bacteria on the GAC, the addition of an oxidising agent (chlorine, chlorine dioxide or chloramine)
to the final water will be minimised. The risk of new taste and odour problems will also be minimised.
To keep the bacteriological quality in the distribution system within the regulations, a careful monitoring by the water company still remains necessary. From time to time a correction of the dosage will be
appropriate.
Ozone dose = 2 mg/l
GAC
O3 + GAC
O3/H2O2 + GAC
elimination effect (%)
0
40
80
concentration of atrazine
after treatment µg/l
1
0.6
0.2
18 000
35 000
105 000
volume of filtered water
(m3) with an atrazine
concentration below 0.1
µg/l per m3 GAC
Table 3: Influence of ozonation on the lifetime of a GAC filter at the WTW Mont Valérien [1]
A further example for this effect is given in Table 3 below:
The combination of ozone and GAC is an ideal and economic solution for the oxidation of organic
compounds in the treatment of surface water to produce a safe potable water quality.
Conclusions
•
•
•
•
•
•
Odour problems can arise in all types of raw waters.
Many of these problems occur either from algae blooms or discharges from industries.
Sometimes substances with weak odour characteristics may be transformed by chlorine to substances with strong odour (phenol ? chlorphenol, THM-precursors ? THM) by the treatment
process (prechlorination).
Newly formed THMs, or already existing THM-levels, will be partially removed by ozone/GAC
filter significantly in a steady state phase.
Ozone is an excellent chemical to eliminate odour problems. If THM precursors are present in the
raw water ozone will reduce them significantly.
The combination ozone/GAC is the most effective technique to eliminate organic matter.
•
•
•
•
•
•
•
The mutagenicity of a raw water will be reduced by ozone as major oxidising agent and even
improved by a downstream GAC filter.
About one third of the DOC can be removed with the combination ozone plus GAC as a steady
state result.
Higher fraction of DOC can be successfully removed if the GAC is operated as adsorptive filter.
This will definitely produce higher operating costs, either by reactivation or by complete exchange
of GAC filter material.
Especially where a GAC filter is operated as a biological active filter, a significant part of the
AOC will be removed.
The risk of bacterial regrowth in the network is therefore minimised.
Furthermore, the lifetime of the GAC can be improved drastically and in some cases no reactivation is necessary, resulting in a substantial saving of operating cost.
The combination ozone/GAC will minimize the addition of any oxidizing chemical to the final
treated water. This will eliminate the problem of creating a new odor problem by such chemicals.
Keywords
Ozone; Activated Carbon; Taste; Odour
References
[1] Yves Richard
Principe d’utilisation de l’ozone en traitement d’eau destinée à la consommation humaine,
Degrémont
Septembre 1994
[2] Jürg Aeppli
Auswirkungen der Reaktivierung von Aktivkohle nach langjährigem Einsatz im
Wasserwerkbetrieb, GWA Sonderdruck
Nr. 1140, Juli 1987
[3] U. Förstner, V. de Haar, F. Jüttner, H. Müller, M. Sonnenborn, H.A. Winkler
Schadstoffe im Wasser, Metalle – Phenole – Algenbürtige Schadstoffe, 1982
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